"Best" is hard to define. Best how and best for what purpose?
I'm partial to Medley's UI, of course, but some of the information visualization work done in Stu Card's group at Xerox PARC in the early 90's was both highly functional and aesthetically pleasing (at least to my eye).
Notable examples include the Perspective Wall, Cone Tree, and InfoGrid. You can find tiny images of some
on this page. The first two ran on Silicon Graphics hardware, the latter on Sun workstations. All the software mentioned was written in Common Lisp, if memory serves. InfoGrid used CLIM, the Common Lisp Interface Manager, as its graphics and interaction framework.
More diligent Web sleuths than I can probably turn up larger images of those and other interfaces created by the group.
- Herb